Apparatus for screening paste solder onto leaded hybrid substrates

ABSTRACT

A stencil for contact printing paste solder onto lead and component pads of the leaded substrate of a SIP is a plate having holes therethrough in the pattern of the pads, a first recess in the solder side of the plate over the area of the plate having component holes, and a second recess in the substrate side of the plate adjacent lead holes that are connected to the second recess by narrow slots dimensioned to receive portions of associated leads. The second recess and lead holes are dimensioned for receiving the lead frame and tines on lead pads. In a printing application, the padded side of the substrate contacts the substrate side of the plate with lead tines on lead pads located in associated lead holes, the thickness of the plate adjacent lead holes being slightly greater than the height that lead tines project above the substrate. The depth of the first recess is selected to make paste solder in component holes adhere to associated component pads when the plate and substrate are separated.

BACKGROUND OF INVENTION

This invention relates to thick film hybrid manufacturing techniques,and more particularly to apparatus for contact printing paste solderonto both component and lead pads of a leaded hybrid substrate in asingle solder screening operation.

The conventional method of manufacturing thick film hybrid circuitsrequires two different solder application steps. In the first step,paste solder is printed onto only component pads on a substrate. Aftercomponent terminals are located in paste solder, the substrate is reflowsoldered to electrically connect the components into electrical circuitpatterns on it. A lead frame is then mechanically attached to thesubstrate by pressing tines of leads over one edge of the substrate sothat some tines are on lead pads. In the second step, the leads and oneedge of the substrate are dipped into molten solder. This manufacturingmethod requires considerable operator time. Although a dip solderoperation can provide a relatively uniform and thin solder coating oflead tines and associated pads, extreme care must be exercised to ensurethat a substrate is dipped deep enough into the molten solder withoutloosening reflow soldered components adjacent the lead pads such thatthey move or fall into the solder bath. In order to reduce the number ofsteps such as storage, cleaning, soldering and inspection that arerequired in fabricating hybrid circuits, an operator will sometimes usea syringe to individually apply paste solder to both component pads andlead pads having tines thereon prior to a single solder reflowoperation. Since this method of dispensing paste solder is operatordependent, it requires considerable operator time. Also, it does notresult in uniform amounts of solder being consistently applied to thepads. This causes non-uniform solder junctions at lead contacts that donot have consistent levels of shear strength. Non-contact printing ofpaste solder using a constant thickness metal stencil having aperturestherein in the same pattern as pads on substrates has also been tried.This non-contact printing technique is similar to applying paste solderwith a syringe and has also been found unsatisfactory.

An object of this invention is the provision of improved apparatus formore consistently applying prescribed amounts of paste solder to leadand component pads on a leaded substrate during a single printingoperation so that lead tines and component terminals may be soldered toassociated pads in a single reflow solder operation.

Another object is the provision of an improved stencil for use inscreening paste solder onto lead and component pads of a leadedsubstrate in a single printing operation.

SUMMARY OF INVENTION

In accordance with this invention, apparatus is provided for contactprinting paste solder onto lead and component pads of a leaded hybridsubstrate in a single screening operation with a stencil that is a flatplate having a plurality of apertures therethrough in the same patternas pads, the thickness of the portion of the plate that is located overcomponent pads being much less than the thickness thereof adjacentapertures for lead pads that have lead tines thereon, the latterthickness being slightly greater than the height that the tines extendabove the lead pads.

DESCRIPTION OF DRAWINGS

This invention will be more fully understood from the following detaileddescription of preferred embodiments thereof, together with drawingfigures that are not drawn to scale, the thickness of some parts beinggreatly enlarged for clarity of illustration. In the drawings:

FIG. 1 is a plan view of an inked substrate 5 having pads 11-14 formedon one side 6 thereof with tines on leads 21 pressed over the edge 8 ofthe substrate and associated lead pads 11 and 12.

FIG. 2 is a right side elevation view of the leaded substrate 5 in FIG.1.

FIG. 3 is an end view of a DIP 4.

FIG. 4 is a plan view of the solder side 37 of a stencil 30 havingcentrally located apertures 31-34 in it in the same pattern as pads11-14 on a substrate 5 there, a first recess 38 in the solder side 37,and a second recess 42 in the substrate side 41.

FIG. 5 is a plan view of the substrate side 41 of the stencil 30 whenonly the stencil is rotated 180° about the centerline A--A in FIG. 4.

FIG. 6 is a section view of the stencil 30 taken along line 6--6 in FIG.4, with a leaded substrate 5 located behind it in the same position itwould have when the side 6 thereof is located adjacent the side 41 ofthe stencil in a printing operation.

FIG. 7 is a schematic representation of a thick film screen printer 60having a stencil 30 attached to the underside of a carriage 68 that isshown in its raised position.

FIG. 8 is a top view of the printer 60 with the carriage 68 rotatedadjacent the platen 62 during a printing operation.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, a relatively simple passive hybridcircuit 3 comprises a ceramic substrate 5 having a plurality ofelectrically conductive pads 11-14 and lines 15-16 formed on the front 6thereof by thick film techniques. The substrate 5 is approximately 0.030inch thick and typically measures from 0.5 inch by 1 inch to 1 inch by 2inches. The lead pads 11 and 12, located adjacent the bottom edge 8 ofthe substrate, are connected to associated component pads 13 and 14 bylines 15 and 16. The pads 13 and 14 are spaced apart on the substratefor purposes of component mounting, e.g., a chip capacitor may beconnected between them. The pads 11-14 typically measure 0.070 inches by0.070 inches. A lead frame 20 is mechanically attached to the substrateby pressing tines 22-24 on leads 21 over the edge 8, with tines 22 beingcentered over and contacting associated ones of the lead pads 11 and 12.The tines 22 are typically 0.010 inch thick and 0.020 inch wide, withthe free ends thereof extending approximately 0.030 inch above the side6 of the substrate. The leads 21 extend below the edge 8 of thesubstrate with the lead surface 26 about 0.010 inch above the side 6 ofthe substrate. Since the leads 21 extend parallel to the surface 6 in aline in a common plane, the circuit in FIGS. 1 and 2 is called a SIP orsingle in-line package 3. A DIP or dual in-line package 4 is shown inFIG. 3 where the two sets of leads 81 and 91 are in associated planesthat are generally orthoginal to the side 6' of substrate 5'. Thesurface 86 of the lead frame 80 here extends only 0.010 inch beyond theedge of the DIP 4.

It is desirable to precisely locate specified amounts of paste solder onpads and tines on lead pads for facilitating making electricalconnection of lead tines and terminals of discrete components (notshown) to associated pads in conductive patterns on hybrid circuits 3and 4 in a single reflow solder operation.

A stencil 30 for contact printing paste solder onto pads of a SIP 3 isshown in FIGS. 4-6. The stencil 30 comprises a thin rectangularly shapedplate having a plurality of apertures 31-34 located in the centerthereof in the same pattern as pads 11-14 on substrate 5 there. Theapertures 33 and 34 are substantially the same size as pads 13 and 14.The apertures 31 and 32 are the same width as and slightly longer thanlead pads 11 and 12. The edges 31B and 32B of lead holes 31 and 32preferably extend slightly below the bottom of pads 11 and 12 but do notextend beyond the edge 8 of the substrate.

The stencil 30 has broad sides 37 and 41 that are flat and parallel, andis preferably made of a 0.032 inch thick sheet of brass that is nickelplated to give it an abrasive resistant surface. The solder side 37 ofthe stencil has a recess 38 milled in it over the area of the substratecontaining component pads. The edge 38' of this recess that is adjacentapertures 31 and 32 is chamfered. In practice, the recess 38 extendsover an area of the stencil that is much greater than that of asubstrate 5. The depth of the slot 38 is approximately 0.015 inch, whichis sufficient to make adhesion between paste solder in apertures 33 and34 and associated component pads 13 and 14 much greater than theadhesion between this solder and walls of apertures 33 and 34 during aprinting operation, as is described more fully hereinafter. Thesubstrate side 41 of the stencil has a slot or recess 42 milled in itover an area that is sufficient for receiving the lead frame 20. It isonly necessary that the recess 42 be deep enough to receive the leadframe 20 in it.

Although the edge 42' of this recess is spaced from the lower edges ofthe lead apertures 31 and 32, the recess 42 extends into the holes 31and 32 as is shown in FIGS. 5 and 6. The walls such as surfaces 31A and32A that define narrow slots in FIG. 5 between associated holes 31 and32 and the large portion of recess 42 extend over the full height ofthis recess, i.e., from the bottom 43 of recess 42 to the side 41 of thestencil. When the substrate 5 and stencil 30 are brought together for aprinting operation (see FIG. 6), the tines 22 are located in associatedlead openings 31 and 32, the adjacent portions of leads 21 that projectabove the side 6 of the substrate resting in the narrow slots 31A and32A so that the side 41 of the stencil is in contact with conductivetraces on the substrate.

The stencil 30 is preferably formed by chemically milling a brass platefrom opposite sides thereof. In this manufacturing process, the plate iscoated with photoresist and sandwiched between a pair of artworknegatives, one carrying the patterns of the recess 38 and openings 31and 32, the other carrying the pattern of recess 42 and all four of theopenings 31-34. After the sandwiched assembly is exposed with light, theartwork is removed so that the photoresist can be developed and washedto remove unexposed resist under the opaque areas of the film thatdefine the recesses 38 and 42 and openings 31-34. The plate is thenplaced in a chemical solution for a prescribed time interval for etchingaway exposed metal from opposite sides of the plate. Where there is noresist on either side of the plate, an opening is etched all the waythrough the plate. Where there is resist on only one side of the plate,an opening is etched only approximately half way through the plate.Where there is photoresist on both sides of the plate, the materialremains 0.032 inch thick after the etching step is completed. It hasbeen found that when the holes 31 and 32 are etched all the way throughthe plate, the plate is about half its original thickness in the areasof recesses 38 and 42, and the holes 33 and 34 are approximately 0.010inch bigger in each direction than the area specified in the artwork.Further etching of the plate will result in larger holes and a thinnermaterial in the area of the recesses.

Reference to FIGS. 4-6 reveals that lead pad openings 31 and 32 areetched through small openings in photoresist on both sides of the plate,whereas in forming the component pad holes 33 and 34 the etchingsolution attacks the side 37 of the plate over the broad area of recess38. Since it is difficult for the etching solution to continuouslycirculate into the small holes 31 and 32, etching in these holes 31 and32 proceeds at a slower rate than does forming of the holes 33 and 34.This difference in etching rates is readily determined byexperimentation and is compensated for by adjusting the size of theopaque area associated with areas for holes 33 and 34 in the originalartwork. The holes 31 and 32 are preferably as large as practicable forpassing a maximum amount of paste solder therethrough since the shearstrength of a solder joint is related to the amount of solder attachinga lead to an associated pad on a substrate. The edge 38' of recess 38 ischamfered at approximately 45° prior to nickel plating to provide asmooth transition area over which a squeege travels in a printingoperation. Finally, a 0.001 inch thick layer of nickel is plated ontothe chemically milled plate to improve its abrasive resistance to pastesolder that is rubbed over it.

FIG. 7 and 8 are schematic representations of a printer 60 for utilizingthe stencil 30 in contact printing paste solder onto hybrid substrates.The printer generally comprises a table having a flat top surface orplaten 62 for supporting a substrate 5 during a screening operation anda carriage 68 for supporting the stencil. The substrate 5 is placed overvacuum holes (not shown) in the platen, with adjacent edges of thesubstrate contacting alignment posts 64 for locating it on the table.The platen 62 has an elongated slot 66 in it for receiving the tines 23and 24 on the back 7 of the substrate so that it will lay flat on thetable. A vacuum is drawn in the holes under the substrate for holding itin place. The stencil 30 has holes 45 around the perimeter thereof (seeFIG. 4) for attaching it to the bottom of the carriage 68 with screws(not shown). Alternatively, the stencil may be bonded or welded to acast aluminum plate that is attached in the conventional manner to thecarriage 68. The printer 60 may, by way of example, be a FORSLUND CermetPrinter, model 35-00, manufactured by Hutchinson Industrial Corporation,Hutchinson, Minn. 55350, or an "ami" model CP-88 Thick Film ScreenPrinter, manufactured by the Presco Division of "affiliatedmanufacturers, inc.," North Branch, N.J.

In operation, the carriage 68 is rotated to bring the stencil 30 intocontact with conductive traces on the side 6 of substrate 5, with tines22 and adjacent portions of leads 21 nested in associated ones of thelead holes and slots in the stencil. A drive mechanism 71 in FIG. 8 isactuated by pressing a switch 73 for causing squeege 75 to move acrossthe top surface 37 of the stencil for forcing paste solder 77 intoopenings 31-34 for forcing it into contact with pads 11-14 and tines 22on the substrate in a single printing operation. Since the openings33-34 are dimensioned so that adhesion between the solder and pads isgreater than that between the solder and walls of these openings, thepaste solder remains on the pads 11 to 14 as the substrate 5 and stencilare separated. After the terminals of a discrete component such as achip capacitor are located in paste solder on pads 13 and 14, allcomponents and leads on the substrate are securely electricallyconnected in the circuit 3 by a single reflow solder operation.

A stencil 30 that was fabricated and successfully utilized in printingpaste solder onto conductive pads and tines of a substrate 5 measuring1.0 inch wide by 0.5 inch long was chemically milled from a 0.032 inchthick brass plate measuring 8.5 inches wide by 10.5 inches long. Therecess 38 measured 2.9 inches wide by 6.0 inches long. The recess 42measured 3.0 inches long by 0.5 inches wide. The component holesmeasured 0.070 inch by 0.070 inch, whereas lead holes were 0.070 inch by0.080 inch. After nickel plating, the recesses 38 and 42 measured 0.015inch deep and the plate adjacent holes 31 and 32 was 0.034 inch thick.

Although this invention is described in relation to preferredembodiments thereof, variations and modifications will be apparent tothose skilled in the art. By way of example, a stencil similar to thatin FIGS. 4-6 may be fabricated with two sets of lead holes for use inprinting paste solder on pads of the leaded DIP 4 in FIG. 3. Since thelead frames of the DIP extend only 0.010 inch beyond the edge 8' ofsubstrate 5', the recess 42 on the bottom of the stencil may be replacedwith an individual recess associated with each lead hole andcorresponding to a short length of the narrow slots 31A and 32A in FIGS.5 and 6. Alternatively, edges of lead holes such as the edges 31B and32B in a stencil for DIP'S or SIP'S may be extended somewhat beyond theedge 8 of a substrate. Also, the stencil 30 may be fabricated bymechanically machining recesses 38 and 42 in opposite sides thereof, andthe stencil may be made of a material other than brass and coated withmaterials other than nickel for increasing the surface hardness thereof.Further, the size and shapes of the openings in the stencil and pads onthe substrate may be varied from that specified here. Also, circuitpatterns on substrates will be more complex than that illustrated inFIG. 1. The scope of this invention is therefore to be determined fromthe attached claims rather than the aforementioned detailed descriptionsof preferred embodiments.

What is claimed is:
 1. A stencil for squeegee screening of paste solderonto a first plurality of component pads in a first area that is spacedfrom a second area containing a second plurality of lead pads, the padsbeing arranged in prescribed patterns on the front of a leaded substratehaving lead tines on associated lead pads that are proximate one edge ofthe substrate, comprising a plate having a thickness that is greaterthan the height that the tines project above the front of the substrate,and having pluralities of first and second openings extending throughthe plate and arranged in associated spaced apart areas in theprescribed patterns of the component and lead pads, respectively, sothat associated oepnings and pads are in correspondence; said secondopenings being dimensioned for receiving associated ones of the tinestherein when the front of the substrate is moved adjacent the back ofthe plate with associated openings and pads in alignment; the front ofthe plate having a first recess in it for decreasing the thicknessthereof over an area that is greater than and that includes the firstarea containing first openings; the back of the plate having a pluralityof adjacent slots therein that are elongated and that extend intoassociated second openings for receiving portions of leads connected toassociated tines when the latter are located in second openings toenable the front of the substrate to be juxtaposed with the back of theplate when they are brought together for a screening operation in whichpaste solder is squeegeed over the front of the plate and the first andsecond openings therein, the depth of the slots being greater than thethickness of lead portions and/or height of tines that are to be locatedherein.
 2. The stencil according to claim 1 wherein the edge of thefirst recess that is proximate the area containing the second openingsis tapered for providing a smooth transition between the front surfaceof the plate within the first recess and the front surface of the platethat is above it and which contains the second openings.
 3. The stencilaccording to claim 2 wherein the plate has a second recess in the backthereof for reducing the thickness of the plate next to second openings,the slots extending between associated second openings and the secondrecess that is dimensioned for receiving leads and a connector barthereon during a screening operation.